1. Field of the Invention
The present invention relates to a ferroelectric memory and a method for testing the same, such as a method for testing the hysteresis characteristics of a ferroelectric capacitor in a memory cell of the ferroelectric memory.
2. Description of the Related Art
A ferroelectric memory includes a memory cell array including memory cells arranged therein, each memory cell including a ferroelectric capacitor. The ferroelectric memory uses the hysteresis characteristics of the ferroelectric capacitor to store data in a non-volatile manner.
Known conventional techniques for testing the hysteresis characteristics of the memory cell in the ferroelectric memory are disclosed in, for example, the following documents.    U.S. Pat. No. 5,661,730    U.S. Pat. No. 5,991,189    U.S. Pat. No. 5,822,237
The procedures for test methods by the conventional techniques include, for example, the following steps (1) to (7).
(1) A function test is performed to compute the yield.
(2) Initial pattern data is written.
(3) A bake process (memory heating process) is performed.
(4) The initial pattern data is read to compute the SS yield (Same State Yield). Specifically, the written initial pattern data is directly read to determine the difference from the written original initial pattern.
(5) A reverse data pattern is written.
(6) The reverse data pattern is read to compute the OS yield (Opposite State Yield). Specifically, the difference is determined between the written reverse data pattern and the read data pattern.
(7) The next pattern data is written.
The procedures (3) to (7) are repeated several times.
In the test, the bake process (3) causes so-called imprint (a hysteresis characteristic curve shift in along the horizontal direction, for example). Steps (4) to (6) determine whether the cell signal is degraded during the imprint.
The bake process (3) is unfavorable because it may allow an imprinted memory to be shipped to customers as a product.
In addition, the bake process takes a long time and thus raises the test cost. Therefore, there is a need for a method for testing the characteristics of the memory cell such as the data holding characteristics in a short period of time without the bake process.